Bug 14445: Silences warn in letter.pl

Bug 14445: Silences warn in letter.pl

When changing Koha module to 'Circulation', there is a warn saying that $code is uninitialized. This patch sets $code to an empty string to silence the warn.

To test:
1) Go to Tools, the Notices & Slips
2) Click 'new notice' (This will trigger warns, but ignore these as they will be corrected in the next patch)
3) Change Koha module to 'Circulation'
4) Notice warn about uninitialized $code variable
5) Apply patch and reload page, change Koha module to 'Circulation'
6) Notice page still works and warns are gone

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>